Electronic devices comprising a layer structure of organic layers have been proposed for different purposes. Among such devices are organic light emitting diodes (OLEDs), organic p-n-diodes, organic photovoltaic devices and alike.
Organic electroluminescent (EL) devices are becoming of increasing interest for applications in the field of displays or lighting sources. Such organic light emitting devices or organic light emitting diodes are electronic devices, which emit light if an electric potential is applied.
The structure of such OLEDs comprises, in sequence, an anode, an organic electroluminescent medium and a cathode. The electroluminescent medium, which is positioned between the anode and the cathode, is commonly comprised of an organic hole-transporting layer (HTL) and an electron-transporting layer (ETL). The light is then emitted near the interface between HTL and ETL where electrons and holes combine, forming excitons. Such a layer structure was used by Tang et al. in “Organic Electroluminescent Diodes”, Applied Physics Letters, 51, 913 (1987), and U.S. Pat. No. 4,769,292, demonstrating high efficient OLEDs for the first time.
Since then, multitudes of alternative organic layer structures have been disclosed. One example being three-layer OLEDs which contain an organic light emitting layer (EML) between the HTL and ETL, such as that disclosed by Adachi et al. in “Electroluminescence in Organic Films with Three-Layer Structure”, Japanese Journal of Applied Physics, 27, L269 (1988), and by Tang et al. in “Electroluminescence of Doped Organic Thin Films”, Journal of Applied Physics, 65, 3610 (1989). The EML may consist of host material doped with a guest material, however neat light emitting layers may also be formed from a single material. The layer structure is then denoted as HTL/EML/ETL. Further developments show multilayer OLEDs which additionally contain a hole-injection layer (HIL), and/or an electron-injection layer (EIL), and/or a hole-blocking layer (HBL), and/or an electron-blocking layer (EBL), and or other types of interlayers between the EML and the HTL and/or ETL, respectively. These developments lead to further improvements in device performance, as the interlayers confine the excitons and the charge carriers within the emission zone and minimize quenching at the interface of the emissive region and the transport layers. They also might reduce the injection barrier from the transport layers into the emission zone, therefore leading to reduced operating voltages of the electroluminescent device.
A further improvement of the OLED performance can be achieved by the use of doped charge carrier transport layers as disclosed in EP 0498979 A1.
For this purpose, the ETL is doped with an electron donor such as an alkali metal, whereas the HTL is doped with an electron acceptor, such as F4-TCNQ. This redox type doping is based on a charge transfer reaction between the dopant and the matrix, releasing electrons (in the case of n-type doping) or holes (in the case of p-type doping) onto the charge carrier transport matrix. The dopants remain as charged species in the matrix, in the case of n-type doping the electron donors are positively charged, in the case of p-type doping the acceptor dopants are negatively charged.
OLEDs using doped charge carrier transport layers are commonly known as PIN-OLEDs. They feature extremely low operating voltages, often being close to the thermodynamical limit set by the wavelength of the emitted light.
One requirement for doped organic layers in OLEDs is that the excitons created within the emission zone have energies high enough to create visible light. The highest energy is needed for an emission in the blue range of the spectrum with a wavelength of 400-475 nm. To allow for such light emission, the electroluminescent material requires a sufficient band gap, which is about the energy of the emitted photons, or higher. It is desirable to choose the energy levels of the HTL and ETL carefully, such that the energy levels match with the emission zone to avoid additional barriers within the OLED device.
The energy levels are frequently identified as HOMO (highest occupied molecular orbital) or LUMO (lowest unoccupied molecular orbital). They can be related to the oxidation potential or the reduction potential of the material, respectively.
In this respect, redox potentials of materials can be provided as a voltage value vs. Fc/Fc+. Fc/Fc+ denotes the ferrocene/ferrocenium reference couple. Redox potentials can be measured for instance by cyclovoltammetry in a suitable solution, for instance acetonitrile or tetrahydrofuran. Details of cyclovoltammetry and other methods to determine reduction potentials and the relation of the ferrocene/ferrocenium reference couple to various reference electrodes can be found in A. J. Bard et al., “Electrochemical Methods: Fundamentals and Applications”, Wiley, 2nd edition 2000.
In case of redox type doping, the energy levels of the acceptor or donor dopants are of importance, too. They can be similarly established by electrochemical methods.
An alternative measure for the oxidation strength of the donor dopant molecule or the HOMO level energy can be ultraviolet photoelectron spectroscopy (UPS). By this method, the ionization potential is determined. It has to be distinguished, whether the experiment is carried out in the gas-phase or in the solid phase, i.e. by investigation of a thin film of the material. In the latter case, solid-state effects such as the polarization energy of the hole remaining in the solid after removal of the photoelectron give rise to deviations in the ionization potential as compared to gas-phase value. A typical value for the polarization energy is around 1 eV (E. V. Tsiper et al., Phys. Rev. B 195124/1-12 (2001).
In order to further improve the performance of OLEDs, such as for example the operation lifetime, stacked or cascaded OLED structures have been proposed, in which several individual OLEDs are vertically stacked. The improvement of the OLED performance in such stacked organic electroluminescent devices is generally attributed to an overall reduction of the operating current density combined with an increased operating voltage, as the individual OLEDs are connected in a row. Such a design leads to lower stress of the organic layers, since the current injected and transported within the organic layers is reduced. Additionally, the stacking of several OLED units in one device allows a mixing of different colours in one device, for example in order to generate white light emitting devices.
The realisation of such stacked or cascaded organic electroluminescent devices can for example be done by vertically stacking several OLEDs, which are each independently connected to a power source and which are therefore being able to independently emit light of the same or of different colour. This design was proposed to be used in full colour displays or other emission devices with an increased integrated density (U.S. Pat. No. 5,703,436, U.S. Pat. No. 6,274,980).
To avoid the need of connecting each of the individual OLEDs within the stacked devices, alternative designs were proposed, in which several OLEDs are vertically stacked without individually addressing each OLED in the unit stack. This can for example be done by placing an intermediate conductive layer with an electrical resistivity lower than 0.1 Ωcm in between the individual OLEDs, consisting of materials such as metals, metal alloys or inorganic compounds (U.S. Pat. Nos. 6,107,734, 6,337,492).
Alternatively, instead of using conductive intermediate layers, the usage of non-conductive charge generation layers (with a resistivity of not less than 105 Ωcm) was disclosed in US2003/0189401 A1.
Even though a stable operation of stacked or cascaded OLEDs can be possible with such conductor or insulator interlayer approaches, the introduction of additional layers such as thin metals is required. Within a production process, these additional layers will cause additional costs, especially as these layers might be produced with different types of evaporation sources than the other organic layers within the stacked OLED devices.
Another approach for the fabrication of cascaded OLEDs was disclosed in EP 1 478 025 A2. Here, a layout using an additional intermediate p-n-junction, formed by an n-type doped organic layer and a p-type doped organic layer with a resistivity of each layer of more than 10 Ωcm in between the individual OLEDs is used to connect OLED units or electroluminescent units. In this approach the interface between the individual OLED units is formed by organic layers, which can be easily processed within an OLED manufacturing process. However, the approach demonstrated still requires the introduction of layers in addition to the layers used within the OLED units. A significant drawback of the approach is the fact, that the organic layers forming the p-n-junction are doped using inorganic elements and molecules with a small atom count. The stacked OLED devices are subject to a rapid breakdown during operation, most likely due to a dopant migration.
For stacked devices only limited lifetime data have been presented demonstrating operational lifetimes suitable for commercial applications.
Some molecular organic dopants, having a somewhat higher atom count such as F4-TCNQ are known in literature, which might be used for p-type doped organic charge transport layers instead of inorganic compounds; however this measure alone does not improve the stability of the stacked OLED devices. Especially the n-type doping could only be achieved by doping organic layers with alkali or earth alkali metals, which act as electron donors within organic layers. Even though there is prior art in which the use of metal salts and metal compounds is described (WO 03/044829 A1), the doping effect in these cases can only be attributed to a cleavage of the salts or compounds that release the metals in an uncharged state. One example is the doping with Cs2CO3, an inorganic salt that upon heating decomposes to release oxygen, CO2 and caesium metal. N-type doping with metals in stacked devices that contain a p-n-interface being driven in reverse direction leads to a rapid breakdown due to metal migration at the junction.
In the document EP-A-1 339 112 in electronic device provided as a cascaded organic light emitting diode comprising a layer structure of organic layers is disclosed, wherein that layer structure comprises a p-n-junction between an n-type doped organic layer provided as an organic matrix material with an n-type dopant and a p-type doped organic layer provided as an organic matrix material with a p-type dopant.
The manufacturing of stacked or cascaded OLED devices with a stable operating lifetime requires the use of interlayers between the OLED units, such as metals, other conductors or insulators. These interlayers are sometimes also referred to as charge generation layers.